In general, fuel cells are power generation systems converting the chemical energy of fuel into electrical energy. For example, a fuel cell converts chemical energy, generated during a chemical reaction between hydrogen fuel supplied to an anode and an oxidizing agent injected into a cathode, into electrical energy.
Such fuel cells are classified as low-temperature type fuel cells and high-temperature type fuel cells according to operating temperature and electrolyte type. A proton exchange membrane fuel cell (PEMFC) is representative of low-temperature type fuel cells and is mainly used for vehicles or the like, while a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), and a solid oxide electrolysis cell (SOEC) using a reverse reaction of SOFC are representative of high-temperature type fuel cells.
A fuel cell 10 is capable of continuously generating power while fuel (hydrogen) and an oxidizing agent are continuously injected thereinto, and by enhancing the utilization rate of fuel in consideration of power generation efficiency and economical efficiency, such a fuel cell may have increased commercial value.
Separators 20 and 30 for the conventional fuel cell 10 include flat plates 22 and 32 providing a flow path in which fuel or an oxidizing agent flows. Intake manifolds 24 and 34 into which the fuel or the oxidizing agent is introduced are provided at one end portions of the flat plates 22 and 32, and exhaust manifolds 26 and 36 from which the fuel or the oxidizing agent is discharged are provided at the other end portions of the flat plates 22 and 32 to be opposite to the intake manifolds 24 and 34. Such separators 20 and 30 for the fuel cell 10 are alternately stacked to form a stack.
Meanwhile, conventional fuel cells 10 are classified according to a variety of schemes on the basis of arrangements of the separators 20 and 30 and the like. For example, representative schemes are a co-flow scheme in which the fuel and the oxidizing agent are introduced in the same direction as illustrated in FIG. 1, a counter-flow scheme in which the fuel and the oxidizing agent are introduced in opposite directions as illustrated in FIG. 2, and a cross-flow scheme in which the fuel and the oxidizing agent are introduced in directions perpendicular to each other as illustrated in FIG. 3.
The flow directions and operating conditions (load, utilization rate and the like) of the fuel and the oxidizing agent in the conventional fuel cell 10 may be determined according to the structures, arrangements, or the like of the separators 20 and 30, and accordingly, it is determined where an electrochemical reaction occurs. Furthermore, due to the structural constraints of the separators 20 and 30 according to the related art, a portion of the fuel cell where the electrochemical reaction occurs is biased toward one side, and accordingly, the temperature gradient is also biased toward one side.
Therefore, the thermal stress distribution in the entire stack of the conventional fuel cell 10 may be asymmetrical, and accordingly, power generation efficiency may be lowered due to the non-uniform flow or biased flow of the fuel or the oxidizing agent. As such non-uniformity is continued, this consequently causes a negative effect on the structural stability of the stack during thermal cycling in the stack.
In this regard, as illustrated in FIGS. 4 through 6, a cross-shift flow scheme has recently been suggested in order to solve the structural constraints in the co-flow scheme, the counter-flow scheme, and the cross-flow scheme.
A fuel cell 50 using a cross-shift flow scheme is provided to reduce non-uniformity of the overall temperature gradient by alternately forming manifold structures in odd-numbered unit cells and even-numbered unit cells.
That is, in separators 60 of the odd-numbered unit cell 51 in which fuel or an oxidizing agent is circulated, some regions of the separators are provided with intake manifolds 62 and 72 into which the fuel or the oxidizing agent is introduced and other regions thereof are provided with exhaust manifolds 64 and 74 from which the fuel or the oxidizing agent is discharged.
In addition, in separators 80 and 90 of the even-numbered unit cell 55 in which the fuel or the oxidizing agent is circulated, intake manifolds 82 and 92 and exhaust manifolds 84 and 94 are provided to intersect those of the separators 60 and 70 of the odd-numbered unit cell 51.
Such a fuel cell 50 using the cross-shift flow scheme may solve the issue of non-uniformity of the temperature gradient to some extent, but portions of the intake manifolds 82 and 92 and the exhaust manifolds 84 and 94 are blocked in corresponding cells, causing a flow distribution in a horizontal direction to be degraded. Thus, the fuel may not be uniformly supplied to the entirety of the cells, whereby the overall performance of the fuel cell 50 may be lowered.
In this regard, a technique for allowing for uniform flow distribution throughout the entirety of the cells through the distributed arrangement of the intake manifolds 82 and 92 and the exhaust manifolds 84 and 94 has been developed.
However, since the conventional fuel cell 50 using the cross-shift flow scheme has sealing members 63, 73, 83 and 93 for sealing between the intake manifolds 62, 72, 82 and 92 and the exhaust manifolds 64, 74, 84 and 94, the sealing members 63, 73, 83 and 93 may interrupt the diffusion of the fuel or the oxidizing agent in the horizontal direction while the fuel or the oxidizing agent is flowing from the intake manifolds 62, 72, 82 and 92 to a reaction surface of the cell, resulting in a flow deviation between the manifold holes and the blocked portions of the manifolds.
Meanwhile, the fuel cell 50 using the cross-shift flow scheme may be modified in order to allow the horizontal diffusion to be more uniform. For example, a separator 60′ of the odd-numbered unit cell 51 in which the fuel or the oxidizing agent is circulated may be modified to have the intake manifold 62 which is disposed in an intermediate position and exhaust manifolds 64a and 64b which are separately formed.
In addition, for example, a separator 80′ of the even-numbered unit cell 55 in which the fuel or the oxidizing agent is circulated may be modified to have the intake manifold 82 which is disposed in an intermediate position and exhaust manifolds 84a and 84b which are separately formed. In this case, the intake manifold 82 and the exhaust manifolds 84a and 84b may be sealed by the sealing members 63 and 83.
Meanwhile, referring to FIG. 9, the conventional fuel cell 50 using the cross-shift flow scheme may create more uniform horizontal diffusion as the number of intake manifolds and exhaust manifolds is increased. However, an area occupied by the sealing members 63 and 83 is also increased and an effective flow area for actual flow is reduced, and accordingly, the overall reaction efficiency and fuel utilization rate may be lowered.
That is, when LH denotes the width of the intake manifold and the exhaust manifold and LS denotes the width of the sealing member, the number of sealing members having the same width may be (n−1) with respect to the n number of manifolds.
The width L of the effective flow area of the fuel cell is represented by the following equation 1:
                    L        =                                            L              H                        ×            n                                                              L                H                            ×              n                        +                                          L                S                            ×                              (                                  n                  -                  1                                )                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Therefore, the conventional fuel cell should be developed to have a structure in which the horizontal distribution of the fuel or the oxidizing agent is improved without reducing the effective flow area L of the fuel cell.